1. Field of the Invention
The present invention relates to satellite surveillance and remote sensing and, more particularly, to a system and method for accurately referencing satellite imagery with respect to geographic locations.
2. Description of the Background
Image Navigation and Registration (INR) systems are widely used in the field of satellite remote sensing, particularly in geostationary weather satellites. A remote sensing satellite collects information about an object or phenomenon within the Field of Regard (FOR) of its sensor. When satellites are placed into geostationary orbit, they are able to view or track a given position on the Earth's surface during the Earth's entire rotational period, semi-indefinitely. Satellites in highly elliptical, polar or near-polar orbits have long dwell times at a given point in the sky during their approach to and descent from apogee, and thus are also able to maintain a single point or points of the Earth's surface within their FOR for an extended period of time. Satellites in low altitude orbits are only briefly able to observe a given position on the Earth with each overpass, but a constellation of such satellites can provide persistent coverage.
Geostationary or highly elliptical, near-polar orbits are commonly used for communications satellites which need a relatively “fixed” position in the sky, as seen from the Earth, in order to maintain continuous contact with a ground location. A geostationary orbit is a circular orbit above the Earth's equator and following the direction of the Earth's rotation, and thus the satellite appears motionless. An example of a highly elliptical orbit is the Molniya satellite system used by the former Soviet Union. Here, satellites were placed into highly eccentric elliptical orbits known as Molniya orbits that allowed them to remain visible to sites in polar regions for extended periods. Geostationary and highly elliptical orbits are especially useful for weather satellites. Geostationary orbit allows a single satellite to monitor changes at a given point or points in the Earth's atmosphere over an entire 24-hour period extended period of time, whereas a highly elliptical, near-polar orbit makes this possible with two satellites. The NASA Tropospheric Emissions: Monitoring of Pollution (TEMPO) mission, for example, operates from a geostationary orbit to measure atmospheric pollution and air quality, including changes in aerosol loads, over a large portion of Greater North America (GNA). Because the instrument remains “fixed” over GNA for the entire mission, it can provide near-real-time air quality measurements to the public during daylight hours. Geostationary weather satellites can also be used to track the movements of weather systems.
In reality, even in the case of a geostationary satellite, orbital motion of the spacecraft causes changes to the satellite's position and orientation relative to the Earth's surface over time. Many of these satellites use INR systems to correct for these changes so that successive projections from the satellite's sensor have the same latitude and longitude on the Earth's surface. INR technology enables the accurate location of an image's individual pixels with respect to geographical coordinates. INR systems rely on sophisticated instrumentation to determine the absolute location and attitude, or orientation, of the orbiting spacecraft. They may also take into account internal configurations such as telescope magnification, the location of a scan mirror which determines the position of the sensor relative to the detector frame, and various other optical alignments.
Currently, state of the art INR systems have the ability to create data products where image pixels are assigned geographic coordinates with errors on the order of the pixel resolution or better. INR systems such as these are used in the Geostationary Operational Environmental Satellites (GOES), operated by the United States National Environmental Satellite, Data and Information Service (NESDIS), and the Meteosat satellites, operated by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), and are termed “exquisite” systems for their ability to geo-locate image pixels with extreme accuracy. However, exquisite systems are very expensive to build and deploy, and only a few mainly government entities, such as NESDIS and EUMETSAT, can afford to own and operate them.
There is, however, an emerging need to furnish high quality INR for lower cost space missions. Such missions may not have sufficient resources to afford their own purpose-built spacecraft with advanced attitude control capabilities, such as the stellar-inertial control system used by the GOES satellites, and may need to fly instead on less capable host spacecraft as secondary payloads. The TEMPO mission, for example, is hosted onboard a commercial geostationary communications satellite, achieving a modest cost.
Accordingly, a need in the art exists for a low cost system capable of measuring orientation and pixel location with a high degree of accuracy. A system and method for low cost, high precision INR by transferring geo-referenced pixel knowledge from an exquisite system to a less sophisticated system is herein presented. The system and method is well suited for use with a remote sensing device operating on a host spacecraft as a secondary payload or as a primary payload in a system where the host spacecraft costs must be kept very low.